Perpendicular magnetic recording, wherein the recorded bits are stored in the generally planar recording layer in a generally perpendicular or out-of-plane orientation (i.e., other than parallel to the surface of the recording layer), is a promising path toward ultra-high recording densities in magnetic recording systems, such as hard disk drives. A common type of perpendicular magnetic recording disk drive uses a “dual-layer” disk. This type of disk drive is shown schematically in FIG. 1. Write current passes through a coil of the write head to generate a magnetic field at the write pole. The dual-layer disk includes a perpendicular magnetic data recording layer on a “soft” or relatively low-coercivity magnetically permeable underlayer (SUL) formed on the disk substrate. The SUL serves as a flux return path for the magnetic field from the write pole to the return pole of the write head. The recording layer has perpendicularly recorded magnetizations or magnetized regions that form a data track, with adjacent regions in the data track having opposite magnetization directions, as represented by the arrows. A sense current passes through the read head, typically a magnetoresistive (MR) read head, such as a tunneling MR (TMR) read head in which sense current passes perpendicularly through the layers making up the head. The magnetic transitions between adjacent oppositely-directed magnetized regions cause changes in electrical resistance that are detectable by the read head as data bits. A shield of magnetically permeable material prevents fields from magnetizations other than the magnetization being read from reaching the read head.
The read head and write head are typically formed as an integrated read/write head patterned on the trailing surface of a head carrier, such as a slider with an air-bearing surface (ABS) that allows the slider to ride on a thin film of air above the surface of the rotating disk, with the direction of the disk relative to the write head being shown by arrow 23. The slider is attached to an actuator arm by a suspension and positioned very close to the disk surface by the suspension. The actuator moves the slider across the disk surface so that the read/write head can access the data tracks. There are typically a stack of disks in the disk drive with a slider-suspension assembly associated with each disk surface in the stack.
The magnetic material (or media) for the recording layer on the disk is chosen to have sufficient coercivity such that the magnetized data bits are written precisely and retain their magnetization state until written over by new data bits. As the areal data density (the number of bits that can be recorded on a unit surface area of the disk) increases, the magnetic grains that make up the data bits can be so small that they can be demagnetized simply from thermal instability or agitation within the magnetized bit (the so-called “superparamagnetic” effect). To avoid thermal instabilities of the stored magnetization, media with high magneto-crystalline anisotropy (Ku) may be required. However, increasing Ku also increases the short-time switching field, H0, which is the field required to reverse the magnetization direction, which for most magnetic materials is somewhat greater than the coercivity or coercive field measured on much longer time-scales. Obviously, H0 cannot exceed the write field capability of the recording head, which currently is limited to about 15 kOe for perpendicular recording.
Since it is known that the coercivity of the magnetic material of the recording layer is temperature dependent, one proposed solution to the thermal stability problem is thermally-assisted magnetic recording (TAMR), wherein the magnetic material is heated locally to near or above its Curie temperature during writing to lower the coercivity enough for writing to occur, but where the coercivity/anisotropy is high enough for thermal stability of the recorded bits at the ambient temperature of the disk drive (i.e., the normal operating or “room” temperature). Several TAMR approaches have been proposed, primarily for the more conventional longitudinal or horizontal recording, wherein the recorded bits are oriented generally in-the-plane of the recording layer.
A “wide-area” heater has been proposed to heat a region of the disk wider than the data track to be recorded. A wide-area heater is relatively easy to implement in a conventional recording head structure and has the additional advantage that it heats the data track very efficiently and thus minimizes the required heater temperature for a given required media temperature. TAMR systems with wide-area heaters include systems that use a laser or ultraviolet lamp to do the heating, as described in “Data Recording at Ultra High Density”, IBM Technical Disclosure Bulletin, Vol. 39, No. 7, July 1996, p. 237; “Thermally-Assisted Magnetic Recording”, IBM Technical Disclosure Bulletin, Vol. 40, No. 10, October 1997, p. 65; and U.S. Pat. Nos. 5,583,727 and 5,986,978. One problem with a wide-area heater is adjacent-track interference (ATI). Because adjacent tracks are also being heated, the stray magnetic field from the write head can erase data previously recorded in the adjacent tracks. Also, even in the absence of a magnetic field, the thermal decay rate in adjacent tracks is accelerated over that at ambient temperature and thus data loss may occur.
A proposed solution for the ATI problem is a “small-area” heater that heats only the data track. U.S. Pat. No. 6,493,183 describes a TAMR disk drive, also for longitudinal recording, wherein the write head includes an electrically resistive heater located in the write gap between the pole tips for locally heating just the data track. A disadvantage of the small-area resistive heater is that due to the relatively inefficient heat transfer the heater temperatures required to reach a desired media temperature are very high. U.S. Pat. No. 6,982,844 describes a TAMR disk drive, also for longitudinal recording, that uses an optical channel with an aperture that emits laser radiation to heat just the data track.
What is needed is a TAMR system and write head for perpendicular magnetic recording.